1. Field of the Invention
The present invention relates to a technique to record and reproduce digital data on a large-capacity recording medium, particularly to a technique to prevent illegitimate recording and reproduction of contents using an illegitimate apparatus.
2. Description of the Related Art
In recent years, since multimedia-related technology has been developed and large-capacity recording media have become available, a system has been getting popular in which digital contents (hereafter, simply referred to as “contents”) that are made of video, audio, and the like are generated and distributed by being stored in a large capacity recording medium such as an optical disc or distributed via a network.
Distributed contents are read with the use of a computer, a reproduction apparatus, or the like, so that they can be reproduced or duplicated.
Generally speaking, an encryption technique is used in order to protect copyrights of contents, in other words, in order to prevent illegitimate use of contents such as illegitimate reproduction and illegitimate duplication. More specifically, a recording apparatus encrypts contents with an encryption key before recording them onto a recording medium, such as an optical disc, and the recording medium is distributed. Only reproduction apparatuses that each have a decryption key corresponding to the encryption key are able to decrypt the encrypted contents read from the recording medium with the decryption key and perform operations including reproduction of the contents.
It should be noted that, when contents are encrypted and recorded onto a recording medium, different methods are used such as (i) contents are encrypted with an encryption key corresponding to a decryption key stored in a reproduction apparatus and then recorded and (ii) contents are encrypted with a key and recorded, and then a decryption key that corresponds to the key is encrypted with an encryption key that corresponds to the decryption key stored in a reproduction apparatus and recorded.
In such cases, it is necessary to strictly manage the decryption key stored in the reproduction key so that it is not disclosed to the outside. There is a risk that such a decryption key may be disclosed to the outside when an illegitimate user illegitimately analyzes the inside of the reproduction apparatus. Once an illegitimate user has discovered the decryption key, there are chances that the illegitimate user may manufacture a recording apparatus or a reproduction apparatus for illegitimately utilizing the contents and illegally sell such apparatuses, or create a computer program for illegitimately utilizing the contents and distribute such a program via the Internet and the like.
In such situations, the copyright holder would want to make sure that the once-disclosed decryption key becomes unable to handle the contents to be provided in the future. A technique to realize this is called a key revocation technique. Patent Document 1 discloses a system that realizes key revocation.
In a conventional key revocation technique, a piece of key revocation information indicating that the key stored in the apparatus is revoked is prestored in an unrewritable area of the recording medium. The apparatus judges whether the key stored in the apparatus is revoked or not using the piece of key revocation information recorded on the recording medium. When the apparatus has a revoked key, it is arranged so that the apparatus is not able to use the recording medium. Also, when a key is newly revoked, the piece of key revocation information is updated, and the updated piece of key revocation information is recorded onto each of recording media that are manufactured after the new revocation of the key. This way, a mechanism is provided in which it is not possible to utilize a new recording medium with a revoked key.
On the other hand, generally speaking, a kind of personal computer peripheral equipment called an optical disk drive is used to read and write contents from and to a recording medium such as an optical disk. In order to achieve compatibility of devices, the method for inputting and outputting content data are standardized as public information and are not made secret normally. Thus, it is possible to easily read the contents recorded on a recording medium with the use of a personal computer and the like, and also possible to easily write the read data onto another recording medium. Accordingly, in order to have a system that protects the copyrights of the contents, it is necessary that the system includes an effective function to prevent the user from reading the data recorded on the recording medium so as to write the data onto another recording medium, which is one of the normal actions that any user is able to perform. A technique used to prevent the data read from a recording medium from being written onto another recording medium is called the media bind technique. The Patent Document 2 discloses a mechanism that realizes a form of the media bind.
In order to achieve copyright protection with the use of the conventional media bind technique, a medium identifier that identifies a recording medium is prestored in the unrewritable area of the recording medium, and the encrypted content recorded on the recording medium is encrypted based on the medium identifier. Accordingly, when only the encrypted content is duplicated onto a second recording medium, as the second recording medium has another medium identifier recorded thereon, it is not possible to correctly decrypt the encrypted content based on this other medium identifier.
In order to prevent illegitimate use of contents from being spread, however, there is a demand that various kinds of techniques for preventing illegitimate use should be achieved.
As referenced above, patent document 1 is identified as Japanese Unexamined Patent Application Publication No. 2002-281013, patent document 2 is identified as Japanese Patent Publication No. 3073590, and patent document 3 is identified as Japanese Unexamined Patent Application Publication No. 09-160492.